1. Field of the Invention
This invention relates in general to gate valves, and in particular to an improved gate and seat which allows shearing of a wireline while maintaining post-shear seal integrity.
2. Description of the Prior Art
During oil and gas production, wirelines are often lowered through some type of gate valve. Ideally, the wireline is removed from the well before the gate valve is closed. However, due to emergencies, or for other reasons, it sometimes becomes necessary to close the gate valve while the wireline is still located in the well and through the gate valve. In these situations, it is desirable for the gate and seat of the gate valve to be capable of shearing the wireline while retaining post-shear seal integrity.
Also of importance in gate valves used in the oil and gas industry is the durability of the sealing surfaces. Since the valves are opened and closed often, and usually under severe operating conditions, the durability of the sealing surfaces is important to avoid leakage and the necessity of having to frequently perform maintenance on the valves.
In the prior art, two types of coatings are generally used on valve gates. Some gate valves are coated with a layer of very hard material such as a carbide material. This type of coating offers great durability. However, use of this type of coating in valves that might have to shear a wireline is not recommended. This type of coating is generally brittle thereby being inherently subject to chipping. Also, this type of coating is generally thin, averaging between 0.003 inches and 0.005 inches. In this section thickness, this coating is not capable of holding an edge while cutting. Furthermore, since this coating is not metallurgically bonded to the substrate material, high shear stresses are developed at the coating-substrate interface which promote cracking of the coating. Cracking or chipping of the coating is not desirable because it reduces sealing efficiency, thereby requiring replacement of the gate more frequently.
Since coatings of very hard materials such as carbides are not ideal for wireline cutting applications, wireline shearing gates have been typically hardfaced with a hard ductile material such as Stellite or Colmonoy to provide protection against chipping when used for shearing. However, large areas are sometimes difficult to coat with these materials without cracking of the coating. Also, such ductile materials have markedly inferior wear characteristics compared to carbides and are easily scratched or otherwise damaged.
Because of the above problem with coating or hardfacing gates with either only an extremely hard material or only a more ductile material, prior art gate valves have not been suited for shearing wireline while retaining post-shear seal integrity.